How to align a cluster of points?
You can achieve this by scaling the points along a perpendicular (Normal) axis:

Via Keyboard:


Alt+Space: Change Transform orientation to Normal
s: Scale selected
z, z: The axis to scale on; the first 'z' specifies the global axis. Pressing again specifies local.
0: The amount to scale by
Enter: Confirm the action


Via Mouse:



Then you need to left click anywhere in the scene to confirm and modify the properties of the tool.



And you will get all the points aligned.